The invention relates to scintillating fiducial patterns and the methods of using and fabricating same.
In electron-beam lithography (EBL), a focused beam of electrons 5 is used to define a pattern 10 in electron-sensitive resist 15, which is on a substrate 20 as shown in FIG. 1A. The pattern is defined in the resist by scanning the electron beam over the resist where the pattern is to be defined. To define the pattern, the electron-beam dose, or current, must be high enough such that when the resist 15 is subsequently developed, the defined pattern 10 will be topographically distinguished as shown in FIGS. 1B and 1C, which depict negative and positive resist, respectively. Typically the sizes of patterns defined are less than 100 .mu.m and can be as small as 10 nm. As an alternative, this type of lithography could also be done with a focused beam of ions, or a focused beam of photons. In that case, the resist must be sensitive to the ions, or photons used.
A method of spatial-phase-locked electron-beam lithography (SPLEBL), or energy beam locating, has been proposed as an improvement to conventional EBL, as described in U.S. Pat. No. 5,136,169. The efficacy of SPLEBL has recently been demonstrated by Wong et al., J. Vac. Sci. Technol. B, Vol. 13, No. 6, p. 2859 (1995). With SPLEBL, the accuracy and precision with which patterns can be defined and placed on the substrate is significantly improved.
In conventional EBL, the patterns are written on a blank substrate and the pattern size and location is defined with respect to mirrors which are mounted on a stage which holds the substrate. The substrate-holding stage can move in two dimensions, X.sub.s and Y.sub.s, The positions of the mirrors are determined with two precise interferometers, one for X.sub.s and one for Y.sub.s. This is referred to as indirect referencing.
In SPLEBL, direct referencing is employed by placing a fiducial grid 230 on an electron-sensitive resist layer 215 provided on a substrate 220, as shown in FIG. 2. Alternatively, the grid could be placed in close proximity (within millimeters) to the resist/substrate composite. The fiducial grid essentially defines a Cartesian coordinate system on the substrate, to which all patterning is referenced. The grid is of high quality: the spatial period is nearly constant everywhere on the substrate or has known distortion, and the axes of the grid (X',Y') 240, are orthogonal. The spatial period, .LAMBDA., of the grid is typically less than 100 .mu.m, and the thickness of the grid is typically less than 50 nm.
As an electron beam 300 scans across the substrate, as shown in FIG. 3B, the interaction of the beam with the fiducial grid gives rise to a modulated signal 310 as shown in the graph of FIG. 3A. This signal may be used to position the beam with respect to the fiducial grid. Since the scanning beam is positioned with respect to the grid, then all patterns are directly referenced to the grid, which is on, or in close proximity to, the resist.
The fiducial grid on the resist/substrate composite, which is used for SPLEBL, has been described typically as being fabricated from islands of resist on a thin layer of metal, or islands of metal over the resist. When an electron beam is incident on any material, secondary electrons (SE) are emitted, which have energies below 50 eV. The number of SE emitted depends upon the type of material. So, as the electron beam scans across a grid of metal islands on resist, or resist islands on metal, the number of SE emitted varies from metal island to resist. Consequently the SE signal 310 is modulated, as shown in FIG. 3A.
The quality of the modulated signal from the fiducial grid must be high for accurate and precise pattern definition and placement in SPLEBL or energy beam locating. One measure of signal quality, is the signal's contrast, C, which is defined as EQU C=S.sub.max /S.sub.min
where S.sub.max is the maximum value of the signal and S.sub.min is the minimum value of the signal, as indicated in FIG. 3A. Another measure of the signal's quality is the amount of amplitude noise on the signal. Amplitude noise is random fluctuations in the signals amplitude, and is indicated in FIG. 3A. If the amplitude noise is large and the signal's contrast is small, then it is difficult to detect the modulation produced by the fiducial grid. This is typically the case in SPLEBL, when secondary electrons are used.
A disadvantage with SE detection of the fiducial grid, as just described, is that all materials emit secondary electrons. And the maximum ratio in SE yield between any two materials is typically less than 2. Consequently, the maximum contrast from any fiducial grid will be typically less than 2.